Light traveling in optical networks often suffers from chromatic dispersion, or polarization mode dispersion, which can lead to degradation of signals and unacceptably high error rates. In order for such networks to function, digital signals are converted into electrical signals, and back into fresh optical signals, at intervals short enough so that error rates are acceptably small. But such optical-electrical signal converters are expensive, and may dominate the cost of the network, and may slow it down. Methods of optically compensating for dispersion can help to reduce such costs, and/or increase speed, by allowing optical signals to propagate for greater distances before they become degraded.
Chromatic dispersion in optical networks, i.e. group delay which varies with frequency, is often compensated for by dispersion compensation fibers, as described, for example, in the article on “Dispersion Compensation” in the online Encyclopedia of Laser Physics and Technology, http://www.rp-photonics.com/dispersion_compensation.html, the disclosure of which is incorporated herein by reference. Such fibers generally produce dispersion compensation that is a fixed function of frequency, depending on the composition of the fiber, even if the requirements for dispersion compensation are changing in time, as occurs, for example, in reconfigurable optical networks. The dispersion compensation as a function of frequency generally provides a good match to the required dispersion compensation only over a limited bandwidth, because there are only a limited number of parameters, in the composition of the fiber, that can be used to adjust the dispersion as a function of frequency. This limits the bandwidth that can be used, and may limit the use of a given dispersion compensation fiber to only a single channel. Dispersion compensation fibers generally do not provide compensation for polarization mode dispersion. They may also be expensive and bulky, since a long length of fiber, as much as several kilometers, may be needed, and a given fiber can only compensate for a fixed degree of dispersion.
Gires-Tournois (GT) etalons are devices made of a transparent material with finite index of refraction n, with a fully reflecting back surface, and a partially reflecting front surface of reflectivity R, parallel to the back surface. Light entering the front surface reflects multiple times inside the etalon, and the interference of the beam components undergoing different numbers of reflections produces an output beam that has very little attenuation, but has a phase delay that is a nonlinear function of frequency, depending on the relationship of the wavelength of the light to the thickness d of the etalon. The GT etalon has a chromatic dispersion that is a periodic function of frequency, that depends on the index of refraction n, the thickness d, and the reflectivity R. An article at htpp://en.wikipedia.org/wiki/Gires-Tournois_etalon, downloaded on Feb. 14, 2007, the disclosure of which is incorporated by reference, provides more details of how GT etalons work, and describes some of their uses, for example in pulse compression, and in nonlinear Michelson interferometers.